The present invention relates to a perforated cellular sound absorption material and, in particular, to a perforated cellular sound absorption material for providing greater sound absorption within a selected frequency range and the method of making such a perforated cellular sound absorption material.
Cellular, acoustical insulation materials, such as foams, are commonly used as silencers for heating, ventilating and air conditioning systems; as pipe insulation; as sound enclosures; as sound barrier curtains; as duct linings; and as sound control materials in aircraft fuselages, office partitions and other applications where sound control is desired or required. The sound frequencies to be controlled in most of these applications lie within a broad range extending from 0 to about 10,000 Hertz and frequently, lie within a much more limited frequency range contained within that broad range, e.g. 0 to 2,000 Hertz.
In connection with the present invention, tests were performed on various cellular materials to measure the average normal incidence sound absorption coefficients of the materials over a selected frequency range to determine the relative sound absorption properties of the cellular materials. While there is not necessarily a direct correlation between normal and random incidence sound absorption at any given frequency, tests of the average normal incidence sound absorption properties for different cellular materials over a selected frequency range can be used as a means of determining the relative normal and random incidence sound absorption properties of the different cellular materials over the selected frequency range. Hence, measurements of the average normal incidence sound absorption coefficients for different cellular samples, over a selected frequency range or ranges, were used to determine the relative normal and random incidence sound absorption properties of different cellular materials.
When compared to low density, relatively large cell foam materials, high density reticulated foams, such as 8.0 pound per cubic foot reticulated polyurethane foams having very small cell structures and baseline airflow resistances of about 2,000 Rayls, perform relatively well as sound absorption materials for a frequency range extending from 0 to about 10,000 Hertz. When compared to low density relatively large cell foams, test results show that these high density reticulated foams exhibit relatively high average normal incidence sound absorption properties for the frequency range extending from 0 to about 2,000 Hertz and it is believed that these relatively high average normal incidence sound absorption properties are exhibited for the entire frequency range extending up to about 10,000 Hertz. While the high density reticulated foams exhibit good sound absorption properties, these foam materials are relatively expensive, when compared to low density foams, such as low density polyimide and polyurethane foams, and the relatively high weights of these high density reticulated foams can be a major detriment when considering these materials for certain sound absorption applications where weight must be minimized, such as sound insulation for aircraft fuselages. Thus, there has been a need to provide relatively inexpensive, low density, cellular sound absorption materials which out perform conventional low density, cellular sound absorption materials and are comparable in performance to the high density cellular sound absorption materials, such as the reticulated polyurethane foam discussed above.